The present invention relates to joint construction particularly suitable for bicycle frames and the like and, more particularly, to joint construction in bicycle frames in which the components are joined by means other than welding or brazing.
The frames for most lightweight touring or racing bicycles comprise a head tube bracket which includes a sleeve for receiving the handlebar and fork stems, a bottom bracket which includes a sleeve for receiving the pedal crank assembly, and a seat bracket which includes an opening for receiving the seat post of the bicycle seat. These three brackets are joined together by tubes to form a triangularly-shaped frame. The tubes are attached to the brackets typically by welding or brazing.
Since the stresses exerted on such bicycle frames during use are concentrated at the head tube bracket, seat bracket, and bottom bracket, these components are made of steel or other strong, metal alloys. However, in order to reduce the overall weight of the frame, it is desirable to make the connecting tubes of aluminum or other alloys which are not as strong as steel, but are much lighter. The use of dissimilar metals in constructing a bicycle frame makes the brazing or welding together of the components difficult if not impracticable. Consequently, other methods of connecting components have been explored.
For example, a bicycle frame is disclosed in the Kleinebenne Gebrauchsmuster No. G 81 17 517.5 in which tubular members are attached to the head tube, bottom bracket, and seat brackets by an adhesive. Each of the brackets includes sockets into which are pressed, soldered or welded tubular "appendages" having tapered ends. The ends of the tubular members are complementarily tapered and sized to slip over the appendages. The inside walls of the tubes and the outside walls of the tapered appendages have roughened surfaces and are secured together by an anaerobic adhesive. The adhesive extends between the overlapping surfaces of the tubes and appendages and solidifies without actually bonding to the surfaces, thereby forming a mechanical interlock.
In one embodiment of that invention, the tubular appendages are cylindrical in shape, rather than tapered, and are mounted within a cylindrical socket formed on the associated connecting component. The socket includes a squared shoulder. The tube is also cylindrical in shape and sized to telescope over the appendage. The tube has a squared end which abuts the squared shoulder of the socket when the two pieces are joined together.
Although this type of connection is suitable for joining together bicycle frame components made of dissimilar materials, and could conceivably be utilized to join non-metallic tubes to metallic connecting components, there are disadvantages with this structure. For example, as a result of normal and unavoidable differences in manufacturing tolerances, a slight difference in the outside diameters of the shoulder and tube end would result in the presence of a sharp, annular edge which may extend partly or completely about the joint. In addition to being unsightly, the sharp edge may present a hazard to the user of the bicycle.
Unavoidable variances in tolerancing which result in the shoulder and tube end being something other than squared would result in the formation of gaps between the two components when joined. Again, this is undesirable since it detracts from the aesthetic impression of the bicycle and may present a safety hazard. In order to remove such sharp edges, it is necessary to manually grind such edges away, or fill any gaps. These manual steps greatly add to the production cost of the resultant bicycle.
Since dissimilar materials commonly have different coefficients of thermal expansion, a tube end and socket made of dissimilar materials may expand or contract at different rates, causing gaps to form or disappear in the joint. Once the bicycle frame has been painted, this relative movement will cause the paint to crack in the area of the joint. In addition to cracking at the joint caused by thermal expansion, cracks may also appear at the joint as a result of stresses imposed upon the bicycle frame during use, or possibly during attachment of other components to the frame, such as the seat or pedal crank assembly.
Accordingly, there is a need for a frame joint construction for bicycles and the like in which the components of the frame are attached by means other than welding or brazing and which produces a strong and alsthetically pleasing joint. Furthermore, there is a need for joint construction which compensates for differing coefficients of thermal expansion of component parts and does not require manual operations to finish the joint.